Mechanism of glucagon activation of adenylate cyclase in the presence of Mn²⁺

Abstract

For a variety of ligand states, adenylate cyclase activity in the presence of Mn2+ was greater than with Mg2+. Trypsin treatment of intact hepatocytes, under conditions which destroy cell surface glucagon receptors, led to a first order loss of glucagon‐stimulated adenylate cyclase activity in isolated membranes assayed in the presence of Mn2+ whether or not GTP (100 μM) was present in the assays. Arrhenius plots of basal activity exhibited a break at around 22°C, those with NaF were linear and those with glucagon ± GTP (100 μM) were biphasic with a break at around 28°C. It is suggested that Mn2+ perturbs the coupling interaction between the glucagon receptor and catalytic unit of adenylate cyclase at the level of the guanine nucleotide regulatory protein. This appears to take the form of Mn2+ preventing GTP from initiating glucagon's activation of adenylate cyclase through a collision coupling mechanism.

The stimulus-sensitive H2O2-generating system present in human fat-cell plasma membranes is multireceptor-linked and under antagonistic control by hormones and cytokines

“…Indeed, Mn2 , which is active at millimolar concentrations only, could be replaced by micromolar concentrations of the stable guanine nucleotide analogue, guanosine 5'-[y-thio]triphosphate, suggesting that the bivalent cation mimics some of the effects of GTP (H. I. Krieger-Brauer and H. Kather, unpublished work). These findings are in accord with previous reports suggesting that the stimulatory effect of Mn2+ on adenylate cyclase may involve a change in G-protein function [53,54] and may explain why similar Mn2+ concentrations were required for all ligands tested. As pointed out in the Introduction, redox control appears to be a broad regulatory system that could allow cells to adapt to environmental changes and appears also to be involved in signalling by hormones and cytokines [4].…”

Alterations in G-protein expression and the hormonal regulation of adenylate cyclase in the adipocytes of obese (fa/fa) Zucker rats

“…However, there did not appear to be any defect in the catalytic unit of adenylate cyclase itself. This was because, with membranes from either lean or obese animals, similar activities of adenylate cyclase were noted under circumstances when this enzyme was stimulated directly by either the diterpene forskolin [2] or Mn2+ ions, which serve to uncouple regulatory Gproteins from adenylate cyclase as well as activating the catalytic unit directly [2,[31][32][33]. Coupling between the stimulatory Gprotein G. and adenylate cyclase was determined by using GTP or its non-hydrolysable analogue GTP[S], as well as NaF, which activates GDP-bound G.. Again, similar responses were obtained with both membrane preparations at maximally effective concentrations of this ligand.…”

Insulin exerts actions through a distinct species of guanine nucleotide regulatory protein: inhibition of adenylate cyclase

“…When Mn2+ replaces Mg2+ in adenylate cyclase assays, the properties of the catalytic unit are changed at the level of its regulation by guanine nucleotides (Londos et al, 1979;Ross & Gilman, 1980;Limbird, 1981). Indeed Mn2+ can affect the coupling of either stimulatory (Limbird, 1981;Braun et al, 1982;Houslay et al, 1983c) or inhibitory (Hoffman et al, 1981) responses. Here we see that, whereas glucagon can still stimulate adenylate cyclase in the presence of Mn2 , the inhibitory effect of insulin was abolished (Fig.…”